The field of photonic crystals (PCs) has for some years received considerable attention from researchers in both academia as well as industry. Such materials are of interest because they interact with visible light through a periodic spatial modulation in their refractive index. In particular, the periodic modulation of refractive indices found in PCs result in the selective reflection of light having wavelengths that correspond to the periodicity of this modulation. An interaction of interest occurs when this periodicity of the refractive index of a PC is comparable to the wavelength of visible light (Arsenault et al., Adv. Mater. 2003, vol. 6 p. 503). This results in an interaction with light that is detectable by the naked eye.
Photonic crystals can be prepared in a one-, two-, or three-dimensional form, with the three-dimensional form to this point in literature representing the more common PC obtainable from bottom-up methodologies. A one-dimensional PC typically comprises periodically alternating layers having different refractive indices. One form of one-dimensional PCs is the distributed Bragg reflector (DBR), also referred to as a Bragg Stack.
DBRs are thin film nanostructures consisting of alternating layers of materials with varying dielectric constants. An example of a DBR is illustrated in FIG. 1. As shown in FIG. 1, each layer boundary causes a partial reflection of an optical wave with many boundaries giving rise to multiple reflections. Provided that the variation in refractive index is periodic, which may be achieved when each layer of equal refractive index is deposited with the same thickness, the many reflected waves can constructively interfere effectively creating a high-quality reflector. The range of wavelengths that are reflected by the DBR is called the photonic stopband. Within this range of wavelengths, light is “forbidden” to propagate in the structure and is instead reflected. Such structures are commonly employed in all branches of optics as frequency selective filters or as antireflective coatings.
In an attempt to impart added functionality to DBRs, variations to the conventional DBR structure have been investigated. Rubner et al. reported the assembly of stacked polyelectrolyte multilayer heterostructures with alternating fully dense and porous regions (Zhai et al., Macromolecules 2004, vol., 37 p. 6113). The pH-gated porosity of the regions that comprised poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) provided a mechanism for achieving reversible refractive-index contrast against fully dense, pH-insensitive regions constructed from PAH and poly(sodium 4-styrenesulfonate) (SPS). The group demonstrated control over the thicknesses of the high and low index regions enabling them to position the reflection band of these one-dimensional photonic crystals (i.e., Bragg Stacks) across the visible spectrum. In addition to the simple demonstration of this “structural color”, they also showed that the reflection-peak wavelength (and therefore the observed color) was sensitive to the condensation of various species in the porous regions. Applications as sensors for trace amounts of solvent vapour and as monitorable drug-delivery systems were discussed. However, these one-dimensional photonic crystals can only reflect a fixed wavelength, as controlled during manufacture of the material. Changes in the reflected wavelength depend on adsorption of various species in the porous regions and cannot be controlled otherwise.
The same group of Rubner et al. reported the observance of structural color from TiO2/SiO2 Bragg reflectors (Wu et al., Small 2007, vol. 3 p. 1445). The nanoparticle DBRs were assembled by polyelectrolyte-assisted layer-by-layer deposition with subsequent thermal treatment of the films to remove the polymer components. The resulting conformal, nanoporous thin-film coatings show the expected narrow-wavelength reflection bands that lead to analyte-sensitive structural color. In addition, the films show favourable superhydrophilicity (antifogging) and self-cleaning properties. Again, changes in the reflected wavelength are dependent on adsorption of analytes within the pores of the material.
Choi et al. reported the preparation of Bragg reflectors consisting of alternating layers of TiO2 and SiO2 mesoporous materials (Choi et al., Nano Lett. 2006, vol. 6 p. 2456). Each layer was prepared by spin coating from the appropriate sol solution followed by a thermal treatment step. The authors demonstrated reversible sensitivity of the structural color of such mesoporous DBRs to the infiltration and removal of analytes in their porous structures. Changes in the reflected wavelength are dependent on adsorption of analytes within the pores of the material.
Currently, tuning of the structural color in such one-dimensional photonic crystal structures has only come from the adsorption of analytes which causes a refractive index change in the pores and a subsequent shift in the Bragg reflection maximum. It would be desirable to expand the functionality of DBRs, for use in other applications.